A conventional method for the production of copper foil for electronic application, i.e., copper-clad laminates for printed circuit boards, typically involves two steps, first, electrodeposition or plating, of a "base" or "raw" foil on a rotating drum-cathode and, second, passing the "base" foil through a "treater" machine, in order to provide the matte side of the foil with a bondable surface suitable for bonding to a polymeric substrate. The latter step is sometimes called the bonding treatment. Traditionally, these two steps are separated by the foil manufacturers, since they seem to be mutually exclusive in that formation of base foil calls for a concentrated, hot copper sulfate/sulfuric acid electrolyte, in order to yield strong, ductile and compact depositions which form the body of the foil, while the bonding treatment usually requires a more dilute and colder electrolyte which yields fragile, powdery deposits whose role is to enhance the true surface area of the matte side of the foil and this enhance the bonding ability of the foil.
In the typical process, the first step, fabrication of the base foil, or "core," a primary objective is to impart to the bulk of the foil the combination of physical, metallurgical and electrical properties desired in the printed circuit industry. Obviously, those properties are determined by the microstructure, which in turn is determined by purity and conditions of the plating process. Typical properties of the core of the foil sought by printed circuit board manufacturers are suitable tensile strength, yield strength, elongation, ductility and resistance to fatigue. Many of the desired properties relate to the maximum load the material may withstand before failure, and are usually derived from stress-strain curves. Similarly, electrical conductivity is considered an important property of copper foil. All these properties of copper foil depend on the foil's microstructure, but particularly on the microstructure of the core of the foil. This microstructure, responsible for foil's properties, is in turn determined by the electrodeposition conditions.
Similar to other materials used in high technology applications, copper foil is a composite; i.e., it has a near-surface region with properties differing from those of the bulk material. Thus, the bulk of the copper foil (core) serves in printed circuit boards as the conductor of electricity. The bonding side of the foil is responsible for promoting a permanent bond to the polymeric dielectric (insulating) substrate or prepreg, e.g., glass fabric impregnated with epoxy resin.
The above-mentioned first manufacturing step utilizes a large cylindrical drum-cathode which rotates, partially immersed in a copper sulfate-sulfuric acid electrolyte. The drum cathode is adjacent to and facing a pair of curved anodes, which may be formed of lead, lead, lead-antimony, platinized titanium, iridium or ruthenium oxides. Both the drum and the anodes are connected electrically to heavy buss-bars to a DC power source, and currents of up to 50,000 amps or more are commonly used. As the drum rotates in the electrolyte, an electrodeposit of copper forms on the drum surface, and as the latter leaves the electrolyte, the electrodeposited copper is continuously stripped from the rotating drum in the form of thin foil, which is lit to size and wrapped around a take-up roll. The outer surface of the drum is usually formed of titanium or other durable metal.
Foil produced in such a process, prior to being treated, is usually referred to as raw foil. The raw foil is pale pink in color and has two distinctly different looking sides--a "shiny side", the side which was plated onto the drum surface and then stripped is quite smooth, while the other side, the side which was facing toward the electrolyte and the anodes, is referred to as the "matte" side, since it has a velvety finish, due to differences in the growth rate of differing crystal faces during electrodeposition of the "base" foil. The matte side surface, at this stage, has a very fine scale micro-roughness and a very specific micro-topography. Viewed under high magnification of a scanning electron microscope, it is composed of peaks and valleys. The peaks are closely packed cones or pyramids, the height, slant, packing and shape of which depend, as is well known, upon closely controlled independent variables of foil thickness, current density, electrolyte solution composition and temperature, as well as upon the type and concentration of the addition agents in the electrolyte and the like.
The surface quality (profile) of the matte side of the base foil determines its suitability for application as a cladding for copper-clad laminates destined for fine line circuitry and the multi-layer printed circuit boards. The criteria of suitability depends upon the quantitative evaluation of the matte side's surface roughness. A variation which gives useful information about the surface is called "Rz," which is the average deviation from the mean line of the five highest peaks and the five lowest valley within the roughness sampling length.
The matte side of the base foil provides the basic shape of the foil surface for embedding in the resin of a substrate to promote adhesion in the copper clad laminates used in the manufacture of printed circuit boards (PCB's).
While the matte side of the foil has a certain micro-roughness it is not nearly good enough to satisfy industry need for foil's bendability. This is why copper foil manufacturers use the second manufacturing step in which a surface bonding treatment is applied to the matte side of the base foil. The term "bonding treatment" is universally used to describe the changing of the morphology of the matte side of the base foil to make it suitable for bonding to laminate resins.
The bonding treatment operation is conducted in machines called "treaters" where in rolls of raw foil are unrolled in a continuous manner and fed into the treater by means of driven rollers (similar to the way in which a web of paper is handled in a printing machine), rendered cathodic by means of contact rollers and passed in a serpentine fashion through a plurality of plating tanks, facing, in each tank, a rectangular anode. Each tank has its own supply of appropriate electrolyte and its DC power source. Between the tanks the foil is thoroughly rinsed on both sides. The purpose of this operation is to electrodeposit on at least one side of the foil, usually the matte side, copper microprojections of complex shape which ensure that the foil will be firmly anchored to the base polymeric materials used in fabricating the copper clad laminates.
High peel strength (the force necessary to pull apart the copper foil and the supporting insulating substrate material) is a characteristic of the highest importance, since the mechanical support of the circuit elements, as well as the current carrying capability of PCB's, is provided by the copper foil--polymer joint. It is essential that the foil is bonded very tightly and securely to the substrate and also that such an adhesive joint can withstand all the manufacturing steps in PCB's fabrication without a decrease of the initial adhesion, which, moreover should remain constant throughout the service life of the PCB.
This bonding operation is carried out in laminating plants and involves heating and cooling cycles. Sheets of copper foil are laid upon sheets of "prepreg" (e.g., glass fabric impregnated with epoxy resin). Both materials are placed in a hydraulic press having heated pressing plates, and the two materials are pressed together under high pressure. At elevated temperatures the resin liquefies and is forced, by the pressure, to flow into the micro-irregularities of the foil surface, and both materials are firmly bonded together and very difficult to pull apart. It is the responsibility of the matte side of the foil to ensure high peel strength.
The characteristics of the bonding side of the finished foil, i.e., the base foil plus treatment, result from the combined effects of the micro-topography of the surface of the base foil (electrodeposited at the drum machine) and the bonding treatment plated upon that surface at the treater machine. Both are equally important.
A preferred bonding treatment is effected by subjecting the matte side of the base or "raw" foil to four consecutive electrodeposition steps. The first consists of the deposition of a microdendritic copper layer which enhances, to a very large degree, the real surface area of the matte side, and thus enhances the foil's bonding ability. It is followed by electrodeposition of an encapsulating, or gilding, layer the function of which is to mechanically reinforce the dendritic layer, and thus render it immune to the lateral shear forces of liquid resins in the laminating stage of the PCB's fabrication. Then, a so-called barrier layer is deposited on the dual-layer copper treatment, after which a stainproofing layer is usually applied.
The purpose of the dendritic deposit is to increase the "true" surface area of the matte side since that property is ultimately responsible for the bonding characteristics of the foil. The shape, height, mechanical strength and the number of dendritic microprojections per surface area which constitute dendritic deposit are the factors instrumental in achieving adequate bond strength of the foil, after all stages of the treatment are completed. The dendritic deposit, the first stage of the treatment, is relatively weak mechanically and given to unacceptable treatment transfer characteristics.
The encapsulating step of the treatment is very important, since it eliminates the foil's tendency toward "treatment transfer" and the resulting "laminate staining" which can cause a decrease of the laminate's dielectric properties. The role of this second treatment stage, is to mechanically reinforce the fragile dendritic layer, by overplating it with a thin layer of sound and strong metallic copper, which locks the dendrites to the base foil. Such a dendrites-encapsulation composite structure is characterized by high bond strength and the absence of treatment transfer. The treating parameters which assure just that are relatively narrow. If the amount of the gilding deposit is too low, the foil will be given to treatment transfer. If, on the other hand, the gilding layer is too thick, a partial loss of peel strength may be expected. These first two layers of the treatment are composed of pure copper, in the form of microscopic, spherical micro-projections.
The dual-layer copper bonding treatment may have electrodeposited thereon a very thin layer of zinc or zinc alloy, a so-called barrier layer. During the fabrication of copper clad laminates destined for PCB's, the zinc-containing layer alloys with the underlying all-copper bonding treatment by the process of heat-accelerated diffusion of metals in the solid state. As a result, a layer of chemically stable alpha brass is thus formed over the surface of the all-copper treatment. Its purpose is to prevent direct copper-epoxy resin contact, and this is why the zinc-containing layer (which during lamination is converted to alpha brass), is referred to as a barrier layer. If the bonding treatment were composed of copper only and subjected to lamination with epoxy resin systems, it tends to react with amino groups of the resin, at the high laminating temperatures. This, in turn, may create moisture at the foil-resin interface, causing the harmful effect of "measling", and possibly delamination. The barrier layer plated over the all-copper bonding treatment prevents these harmful effects.
All three stages of the treatment mentioned above, as is well-known in the art, are effected by means of electrodeposition for the purpose of changing the geometry and morphology of the matte side of the foil and assure the mechanical strength of the surface region. (U.S. Pat. No. 3,857,6781, Yates et al.)
Foil treated as described above may then be subjected to an electrochemical stainproofing which changes the surface chemistry. As a result of this step, the bonding surface is rendered chemically stable. This stainproofing operation removes weak surface films, which can greatly decrease the adhesion of the foil to the substrate, and provides a stable film of controlled thickness, responsible for imparting on the treated surface "durability" of its properties.
While the above-described bonding treatment provides copper foil with very satisfactory properties, it requires expensive equipment and transferring the foil to and between the treater foils.
Various patents disclose techniques for combining the base foil manufacture and bonding treatment deposition into a single operation, for example:
U.S. Pat. No. 3,674,656 discloses a technique for making copper foil as a drum-cathode machine using a secondary anode delivering a relatively high current density to promote high-profile, high-bond matte height. While good bonding ability is achieved, the matte side of the foil, by the patent's own description is "highly roughened", in the form of "tree like growth", and would not satisfy the requirement of low-profile cross-section.
International patent No. W08703915 teaches a technique that combines agitating the electrolyte while using a secondary pulsed current having a current density greater than the limiting current density to produce copper foil having a nodularized outer surface. This technique, however, apparently does not achieve fabrication of low profile foil.
U.S. Pat. No. 5,215,646 (Wolski et al.) and U.S. Pat. No. 5,863,410 (Yates et al.) each disclose a process and apparatus for the production of finished copper foil using a secondary anode, delivering a higher current density than that delivered by the primary anode, to provide a finished copper foil having a high peel strength.
While the latter techniques produce excellent low profile foil, we have found it highly desirable to develop a process and apparatus which enables the production of finished copper foil having matte side surface characteristics which are more uniform from batch-to-batch and from machine-to-machine, and which improve the controllability of the foil-making operation.
International Publication WO 99/16935 discloses a treated electrolytic copper foil where the surface of the shiny side of the raw foil is modified by an electrodeposited copper gilding layer, a copper dendritic layer electrodeposited on the gilding layer, and a second copper gilding layer electrodeposited on the dendritic layer. Further, a second copper dendritic layer may be electrodeposited on the surface of the matte side of the foil and a copper gilding electrodeposited on the second dendritic layer.
U.S. Pat. Nos. 5,437,914, 5,447,619, and 5,779,870 disclose "reverse treated" copper foils for use in the manufacture of multilayer printed circuit boards (MLB's), wherein the foils have a bond-enhancing copper treatment deposited on the shiny side of the foil, and laminates formed by bonding the shiny side of the foil of a polymeric substrate.